1. Field of the Invention
The present invention relates to a semiconductor device using an insulator film, represented by a thin film transistor (referred to hereinafter as “TFT”), and to a process for fabricating the same. The present invention also relates to a process for fabricating, in high yield, a high-performance and reliable insulated gate semiconductor device on an insulator substrate under a temperature as low as 700° C. or even lower, and for fabricating an integrated circuit (IC) by assembling a plurality of such semiconductor devices.
The present device is useful as active matrices of liquid crystal displays, etc., driver circuits of image sensors, etc., and as TFTs of SOI (silicon on insulator) circuits and of conventional semiconductor ICs such as microprocessors, microcontrollers, microcomputers, and semiconductor memories.
2. Prior Art
Conventionally, liquid crystal display devices and image sensor devices are well known as devices using TFTs being integrated on a glass substrate. In general, insulated gate field effect semiconductor devices using thin film transistors are employed in the conventional devices above, and it is also customary to use a silicon oxide film as the gate insulators of those TFTs.
The TFTs using the silicon oxide film as the gate insulator, however, suffer problems such as the leak current ascribed to the pinholes in the gate insulator film, limits in increasing the film thickness (the capacity of a gate insulator depends on the film thickness and permittivity), instability in the required various properties as an insulator film due to the lack of density (that is, the film is too soft), and to the problems attributed to fixed charge such as sodium ions being incorporated in the gate insulator.
Recently, study is made intensively on the process for fabricating an insulated gate semiconductor device (MOSFET) on an insulator substrate. Those ICs having established on an insulator substrate are advantageous in their suitability to high speed drive, because, the ICs having established on an insulator need not suffer stray capacitance. In contrast to these ICs, the operation speed of a conventional IC is limited by a stray capacitance, i.e, a capacitance between the connection and the substrate. The MOSFETs having formed on an insulator substrate and having a thin film active layer is called a thin film transistor (TFT). A TFT can be found in a conventional semiconductor IC, for example, as a load transistor of an SRAM.
Furthermore, some of the recent products, for example, driver circuits for optical devices such as liquid crystal displays and image sensors, require a semiconductor IC to be formed on a transparent substrate. TFTs can be found assembled therein, but the ICs must be formed over a wide area and are thereby required that the TFTs are fabricated by a low temperature process. Furthermore, in devices having a plurality of terminals each connected with semiconductor ICs on an insulator substrate, for instance, it is proposed to reduce the mounting density by forming the first layer of the semiconductor IC or the entire semiconductor IC itself monolithically on the same insulator substrate.
Conventionally, the quality of TFTs have been ameliorated by providing a high performance (i.e., a sufficiently high mobility) semiconductor film by improving the crystallinity of an amorphous or a semi-amorphous film, or a microcrystalline film, by irradiating an intense light such as a laser beam thereto or by thermally annealing those films in the temperature range of from 450 to 1200° C. Amorphous TFTs using an amorphous material for the semiconductor film can be certainly fabricated; however, their application field is greatly limited by its inferior operation speed ascribed to too a low mobility of 5 cm2/Vs or lower, about 1 cm2/vs in general, or by its inability of providing a P-channel TFT (PTFT). A TFT having a mobility of 5 cm2/vs or higher is available only after annealing the structure at a temperature in the range of from 450 to 1200° C. A PTFT can be fabricated only after subjecting the film to such annealing treatments.
However, in a process where a high temperature is required, in particular, only strictly selected substrate material can be used. More specifically, a so-called high temperature process which includes high temperature heating in the range of from 900 to 1,200° C. is advantageous, because it allows the use of a high quality film obtainable by thermal oxidation as a gate dielectric, but the usable substrates were confined to those made from expensive materials such as quartz, sapphire, and spinel, and they were not suited as substrates for large area applications.
In contrast to the high temperature process above, substrate materials can be selected from a wider variety in a low temperature process in which a maximum attainable temperature is 750° C. or lower for the entire process inclusive of a crystallization step using laser irradiating. However, there remains a problem of forming insulator films at a low temperature yet at a favorable step coverage and a high throughput. The insulator films can be deposited at a low temperature by sputtering, however, the process is still inferior considering its poor step coverage and insufficient throughput that results therefrom. Also known is depositing a silicon oxide film at a low temperature and high throughput by chemical vapor deposition (CVD) processes such as plasma CVD, low pressure CVD, and normal pressure CVD, in which a gasified organic material containing silicon atoms (referred to hereinafter as organic silane) such as tetraethoxysilane (TEOS) is used as the starting material. The resulting films, however, are rich in carbon atoms and hydrocarbon groups which develop into clusters to provide trap centers. Accordingly, those films are not suited for gate dielectrics because they fail to provide sufficiently high insulating properties and have too high interfacial level density.
The silicon oxide films using organic silane as the starting material as above cannot be used as-deposited for a material such as gate insulator film in which a sufficiently high electric properties are required. Accordingly, they were used only after subjecting them to an oxidation treatment at 700° C. or higher for a long duration of time. Such a heat treatment damages the substrate and impairs the throughput.